1. Technical Field of the Invention
This invention relates to a shielded strip line device to be mounted on a circuit board or an electronic substrate and a method of manufacture thereof, and particularly relates to a shielded strip line device, which is favorably adapted to high speeds and high frequencies, mainly for use as a noise filter bypass device or power decoupling device, and a method of manufacturing such a shielded strip line device.
2. Description of the Related Art
With the progress of scientific technologies, the making of electronic equipment more compact and higher in performance is being demanded. This is achieved, for example in switching power supplies and digital signal processing circuit parts, by making the clock frequency higher. However, this causes an increase in the high frequency current that flows through the circuit, in particular the power supply circuit, causing significant increase in electromagnetic radiation and lowering of signal quality and thus placing more difficult demands on the performance of noise filter bypass devices and power decoupling devices.
As power decoupling devices for high-speed digital circuits, ceramic capacitors, formed by laminating a plurality of layers of ceramic material having a thin metal film deposited thereon, and solid-state electrolytic capacitors, having a porous formed body of a valve metal, such as tantalum or aluminum, etc., as an anode, the oxide film of this porous formed body as a dielectric, and a conductive polymer as a solid-state electrolyte, have been developed priorly.
As an example of a solid-state electrolytic capacitor, Japanese Patent Publication No. Hei-4-56445 (Japanese Unexamined Patent Publication No. Sho-60-37114) discloses a solid-state electrolytic capacitor having polypyrrole or an alkyl-substituted form thereof disposed as a solid-state electrolyte on a dielectric oxide film. Also, Japanese Unexamined Patent Publication No. Hei-3-35516 discloses a solid-state electrolytic capacitor, with which polyaniline is formed as a solid-state electrolyte on a dielectric oxide film, and a method of manufacturing such a solid-state electrolytic capacitor. With each of these capacitors, a conductive polymer that is 100 times or more higher in conductivity in comparison to prior capacitors is used as the solid-state electrolyte. These capacitors are thus small in equivalent serial resistance and, in comparison to prior capacitors of the same capacitance, exhibit effects up to a high frequency range that is 100 times or more higher in frequency. However, even these capacitors increase dramatically in impedance and cannot meet recent demands as filter bypass devices and power decoupling devices in high frequency ranges in the excess of 10 MHz.
Meanwhile, filter arrangements for accommodating high frequencies are also being examined. Japanese Unexamined Patent Publication No. Hei-6-53046 discloses a surface-mounted noise filter, comprising a meandering conductor and a ground conductor, each of which is sandwiched by ceramic dielectric sheets. FIG. 1 is a sectional view, which shows the arrangement of this prior-art surface-mounted filter. As shown in FIG. 1, with this prior-art surface-mounted filter, a first dielectric sheet 110, second dielectric sheet 120, and third dielectric sheet 130, which are, respectively, rectangular form, are laminated to form a laminated body 153. A first signal electrode 151 and a second signal electrode 152 are attached respectively to each of the pair of mutually opposing end faces among the end faces parallel to the direction of lamination of laminated body 153.
A first internal conductor 111, second internal conductor 112, and meandering conductor 115, which are used for signal transmission, are disposed between first dielectric sheet 110 and second dielectric sheet 120. First internal conductor 111 is connected to first signal electrode 151, second internal conductor 112 is connected to second signal electrode 152, and meandering conductor 115 is connected between first internal conductor 111 and second internal conductor 112. A ground conductor 125, which opposes meandering conductor 115, is disposed between second conductive sheet 120 and third conductive sheet 130, and ground conductor 125 is connected to a pair of ground electrodes (not shown). These ground electrodes are attached to a pair of end faces, among the end faces of laminated body 153 that are parallel to the direction of lamination of laminated body 153, to which first signal electrode 151 and second signal electrode 152 are not attached. An inductance is formed at meandering conductor 115 and a capacitance is formed across meandering conductor 115 and ground conductor 125. A noise filter, which combines an inductance device and a capacitance device, is thus formed and noise filter that is excellent in noise absorption characteristics at high frequencies is thus obtained. With this surface-mounted filter, an electric signal that is input from first signal electrode 151 is filtered by passage through first internal conductor 111, meandering conductor 115, and second internal conductor 112 and output from second signal electrode 152.
However, this prior art has the following problem. Though capacitors, having an abovementioned conductive polymer as a solid-state electrolyte, are used in various applications as capacitors that can be used up to a high-frequency range, even with such capacitors, the impedance increases drastically in a high-frequency range in the excess of 10 MHz. Thus under operation at a clock frequency of several hundred MHz, which is generally implemented in digital circuits, the characteristic assumed for a signal generating circuit, that is, the characteristic that the power impedance is infinitely close to zero regardless of frequency cannot be realized as long as such a capacitor is used. As a result, these capacitors cannot meet recent demands as filter bypass devices or power decoupling devices. Also, though surface-mounted filters for the purpose of noise elimination have also been developed, these cannot realize an infinitely low impedance value and are thus limited in use as substitutes for capacitors. It is thus difficult to realize low impedance especially in high-frequency regions of 100 MHz or more with such filters.
An object of this invention is to provide a shielded strip line device, which is low in impedance, especially in high-frequency regions of 100 MHz or more, and is favorably adapted to high speed and high frequencies, mainly for use as a bypass device for a noise filter or as a decoupling device.
A shielded strip line device according to the present invention comprises a metal member formed of a valve metal through which high-frequency current flows and with which the cross-sectional shape orthogonal to the direction in which the high-frequency current flows is practically constant in the current direction, a dielectric oxide film, formed on the surface of this valve metal member, and a conductive layer, provided so as to sandwich the dielectric oxide layer and surround the metal member.
With this invention, by making the metal member have a transmission line structure with which the cross-sectional shape orthogonal to the direction of current is constant or fixed, the electromagnetic field within the metal member is made uniform even when the current that flows inside the metal member is a high-frequency current and the frequency dependence of the characteristic impedance is thus made small. Also by forming the metal member from a valve metal, a dielectric oxide film can be formed on the surface of the metal member. A valve metal is a metal that forms a dielectric oxide film on its surface. Furthermore, by providing a conductive substance layer (conductive layer) so as to surround the metal member, a shielded strip line, which is a form of transmission line structure, is realized. A strip line is a structure having conductive layers disposed above and below a signal line and a shielded strip line is a structure with which these upper and lower conductive layers are connected to each other at the sides of the signal line. The magnetic flux that leaks from the sides of the metal member that is the signal line can thereby be shielded, thus lowering the characteristic impedance of the device further. As a result, electric signals input into the metal member can be filtered by means of the dielectric oxide film and the conductive layer over a wide frequency range. A line device adapted for high speeds and high frequencies can thus be realized.
The abovementioned cross-sectional shape may be rectangular, circular, or annular. The absolute value of the characteristic impedance of the device also depends on the cross-sectional shape of the metal member. In a case where the abovementioned cross-sectional shape is rectangular, the shape of the abovementioned metal member will be a rectangular parallelepiped shape, such as a flat plate-like shape. In a case where the abovementioned cross-sectional shape is circular, the shape of the metal member will be columnar. In a case where the abovementioned cross-sectional shape is annular, the shape of the metal member will be cylindrical. The abovementioned cross-sectional shape only needs to be practically rectangular, circular, or annular.
The abovementioned valve metal preferably comprises one type or two or more types of metal selected from the group consisting of aluminum, aluminum alloys, tantalum, tantalum alloys, niobium, and niobium alloys. A uniform and stable dielectric oxide film that is high in dielectric constant can then be formed by oxidizing the surface of the metal. As a result, a stable shielded strip line device of excellent volumetric efficiency can be obtained readily.
Furthermore, the abovementioned conductor layer preferably comprises a conductive polymer. A conductor layer of high conductivity that closely contacts the dielectric layer formed on the surface of the metal member can then be formed even when the surface of the metal member is expanded by etching, etc. A shielded strip line device that can be used up to a high frequency range can thus be obtained readily.
In particular, the abovementioned conductive polymer preferably comprises one type or two or more types of substance selected from the group consisting of polypyrrole, polythiophene, and polyaniline. A conductor layer that is excellent in environmental stability and is stable to a temperature of at least 100xc2x0 C. can then be formed. As a result, a shielded strip line device that is excellent in stability and durability and can be used up to a high frequency range can thus be obtained readily.